Obviously, this shouldn't be a concern if preview is turned off - I
have no argument that selecting the next message would be proper
there. If preview is on... maybe we can select the next message
without previewing it (i.e. a right-click on an unselected message).

I think we should stay consistent here and not change behavior based
on whether previews are enabled or not. Especially since this can be
changed on demand and is not a pure preference.

I see now that shift-delete does what I want here, but I still think
that when using a trash folder, the shift-delete behavior should be
the default - otherwise you are constantly losing your place in the
mailbox in terms of keyboard nav.

The next message is not selected on purpose - the reasoning being that
we want to avoid loading the preview of a message (a potentially
expensive call on the server side) unless we are absolutely certain
the user wants to view that message. Additionally, IIRC, user studies
from large organizations (e.g. SAPO) indicated that many users are
"afraid" to click on a message that they know is SPAM for fear that by
loading the preview, there computers might be infected/affected.
Quite honestly, it is not an unreasonable fear and I agree with the
conclusion that we should only show previews if a message is
explicitly clicked on (this fear is the main reason why a right-click
does not automatically load the preview).

Obviously, this shouldn't be a concern if preview is turned off - I
have no argument that selecting the next message would be proper
there. If preview is on... maybe we can select the next message
without previewing it (i.e. a right-click on an unselected message).

I see now that shift-delete does what I want here, but I still think
that when using a trash folder, the shift-delete behavior should be
the default - otherwise you are constantly losing your place in the
mailbox in terms of keyboard nav.

When deleting messages from the viewport and using a trash folder, the
selection is lost. I'd like to see deletion be treated as a down
arrow/next message action from the point of view of the selection. If
there are no messages after the one(s) being deleted, then the
selection should move up instead. Similarly to the enhancements for
hitting up/down when multiple messages are selected, if multiple
messages were deleted, the up/down behavior should be from the
top/bottom of the selection, respectively.